Glitch preventing input/output circuits

ABSTRACT

Circuits and methods for preventing glitch in a circuit are disclosed. In one example, a circuit coupled to an input/output pad is disclosed. The circuit includes: a first level shifter, a second level shifter, and a control logic circuit. The first level shifter is configured for generating a data signal. The second level shifter is configured for generating an output enable signal. The first and second level shifters are controlled by first and second power-on-control signals, respectively. The control logic circuit is coupled to the first level shifter and the second level shifter.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/901,418, filed on Jun. 15, 2020, which is a continuation ofU.S. patent application Ser. No. 15/965,875, filed on Apr. 28, 2018, nowU.S. Pat. No. 10,686,438, issued on Jun. 16, 2020, which claims priorityto U.S. Provisional Patent Application No. 62/551,467, filed on Aug. 29,2017, each of which is incorporated by reference herein in its entirety.

BACKGROUND

Integrated circuit chips (IC chips) or semiconductor die are typicallyencapsulated in a package to protect the circuitry formed on thesemiconductor die from external elements. An IC chip includes bond padsformed thereon. Bond wires, or other electrical connection means, areused to electrically connect the bond pads to corresponding pins orleads of the integrated circuit package. The bond pads can be power padsfor power supply voltages connections and input/output (I/O) pads forconnecting to input and output signals of the integrated circuit. An I/Ocircuit is a circuit coupled to an I/O pad of a chip and configured tocommunicate input and/or output signals with other chips in theintegrated circuit system.

A glitch is an undesired transition state that occurs before a signalsettles to its intended value. Glitching poses a critical issue for anI/O circuit, e.g. during power ramp-up of the circuit. A key factorimpacting the glitch issue in an I/O circuit is the signal sequence of adata signal and an output enable signal in the I/O circuit. In aconventional method, the signal sequence is controlled by system levelsignals, which is not suitable for high-speed circuit operations. Inanother conventional method, a power-on-control (POC) mechanism isrequired to be added externally to control the I/O circuit in tristateand avoid crowbar-currents during power ramp up. For this external POCmethod, customers of the integrated circuit have to modify their systemdesign to control POC behavior by themselves, which is a huge effort forthe customers. As such, existing I/O circuits are not entirelysatisfactory in terms of glitch prevention.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that various features are not necessarily drawn to scale. In fact,the dimensions and geometries of the various features may be arbitrarilyincreased or reduced for clarity of discussion. Like reference numeralsdenote like features throughout specification and drawings.

FIG. 1 illustrates an exemplary block diagram of a circuit coupled to aninput/output pad, in accordance with some embodiments of the presentdisclosure.

FIG. 2A illustrates an exemplary circuit behavior during power ramp-up.

FIG. 2B illustrates another exemplary circuit behavior during powerramp-up.

FIG. 3 illustrates an exemplary gating circuit in an input/outputcircuit, in accordance with some embodiments of the present disclosure.

FIG. 4 illustrates an exemplary layout of a gating circuit in aninput/output circuit, in accordance with some embodiments of the presentdisclosure.

FIG. 5 illustrates another exemplary gating circuit in an input/outputcircuit, in accordance with some embodiments of the present disclosure.

FIG. 6 illustrates an exemplary circuit behavior during power ramp-up ofan input/output circuit, in accordance with some embodiments of thepresent disclosure.

FIG. 7 illustrates an example of detailed circuit behaviors during powerramp-up of an input/output circuit, in accordance with some embodimentsof the present disclosure.

FIG. 8 illustrates an exemplary block diagram of another circuit coupledto an input/output pad, in accordance with some embodiments of thepresent disclosure.

FIG. 9 is a flow chart illustrating an exemplary method for preventingglitch in a circuit coupled to an input/output pad, in accordance withsome embodiments of the present disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure describes various exemplary embodiments forimplementing different features of the subject matter. Specific examplesof components and arrangements are described below to simplify thepresent disclosure. These are, of course, merely examples and are notintended to be limiting. For example, the formation of a first featureover or on a second feature in the description that follows may includeembodiments in which the first and second features are formed in directcontact, and may also include embodiments in which additional featuresmay be formed between the first and second features, such that the firstand second features may not be in direct contact. In addition, thepresent disclosure may repeat reference numerals and/or letters in thevarious examples. This repetition is for the purpose of simplicity andclarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. Terms such as“attached,” “affixed,” “connected” and “interconnected,” refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Reference will now be made in detail to the present embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

The present disclosure provides various embodiments of glitch preventingI/O circuits and methods for preventing glitches in I/O circuits. Insome embodiments, a gating circuit is provided to control the signalsequence of a data signal and an output enable signal in the I/Ocircuit, to prevent any glitch from occurring especially when a corecircuit to which the I/O circuit is coupled and the I/O circuit arerespectively powered up or turned on, i.e. to ensure no glitch during apower ramp-up process of the I/O circuit. For example, the I/O circuitincludes a first level shifter configured for generating the datasignal, a second level shifter configured for generating the outputenable signal, and a control logic circuit configured for driving aninput/output pad to a desired voltage level based on the data signal andthe output enable signal.

To ensure the signal sequence of the data signal and the output enablesignal, two separate (first and second) power-on-control signals areused to control the first and second level shifters, respectively, inaccordance with some embodiments. The gating circuit can generate thedata signal and the output enable signal, and can control the signalsequence of the data signal and the output enable signal to ensure that:the first power-on-control signal enables the first level shifter togenerate the data signal before the second power-on-control signalenables the second level shifter to generate the output enable signal.In this manner, no glitch will appear during the power ramp-up processof the I/O circuit because the data signal is ready and active beforethe output enable signal is ready and active.

In one embodiment of the present teaching, the gating circuit includes afirst transistor gated by the data signal, a second transistor gated bya signal that is a logical inversion or complement of the firstpower-on-control signal, and a first inverter configured to generate thesecond power-on-control signal based on outputs of the first transistorand the second transistor. The structure of the gating circuit ensuresthat the second power-on-control signal is generated based on whetherthe data signal has reached a stable logic state, which avoids glitchesin the I/O circuit.

In another embodiment of the present teaching, the gating circuitincludes a series circuit having an even number of inverters connectedin series. The series circuit receives the first power-on-control signalas an input signal and generates the second power-on-control signal asan output signal. The even number is designed to be large enough toensure that the second power-on-control signal is generated after thedata signal has reached a stable logic state.

The present disclosure is applicable to any I/O circuit, e.g. a generalpurpose input output (GPIO) circuit, in an integrated circuit chip. Insome embodiments, the power-on-control signal is generated by aninternal block of the integrated circuit chip, such that customers ofthe integrated circuit chip do not need to add extra effort to controlthe signal sequence. In addition, the gating circuit disclosed hereinmerely occupies a small area compared to the total area of the I/Ocircuit, which has minimum or no area impact to the I/O circuit.Furthermore, the gating circuit does not have performance impact tonormal operation of the I/O circuit, while avoiding glitches duringpower ramp-up of the I/O circuit. According to some embodiments of thepresent teaching, the I/O circuit includes one or more additional gatingcircuits configured to generate more power-on-control signals andcontrol a signal sequence of all power-on-control signals based on apredetermined design, to ensure a desirable timing sequence of signalsin the I/O circuit.

FIG. 1 illustrates an exemplary block diagram of a circuit 100 coupledto an input/output pad 190, in accordance with some embodiments of thepresent disclosure. In accordance with one embodiment, the circuit 100may be part of a chip in an integrated circuit system. The chip cancommunicate with other chips in the system via the I/O pad 190controlled by the I/O circuit 100. As shown in FIG. 1, the circuit 100includes two portions, a core portion 110 having a first voltage domainand an I/O portion 120 having a second voltage domain.

In this example, the core portion 110 includes a first buffer 111 thatreceives an input data signal I and generates a smoothed data signalwith a same voltage level as the input data signal I. The input datasignal I may be generated by a core circuit connected to the firstbuffer 111. The core circuit (not shown) performs a core function of thechip according to customer design. The core circuit generates the inputdata signal I to output data to another chip via the I/O pad 190. Theinput data signal I generated by the core circuit has a voltage within acore domain, e.g. 0 V to 0.75 V.

The core portion 110 in this example also includes a second buffer 112that receives an output enable signal OE and generates a smoothed OEsignal with a same voltage level as the output enable signal OE. Theoutput enable signal OE is also generated by the core circuit connectedto the second buffer 112. The core circuit generates the output enablesignal OE to control the data output to another chip via the I/O pad190. The output enable signal OE generated by the core circuit has avoltage within the core domain, e.g. 0 V to 0.75 V.

The I/O portion 120 in this example includes a first level shifter 121that is coupled to the first buffer 111 and receives the smoothed datasignal from the first buffer 111. The first level shifter 121 can shiftthe smoothed data signal from the core voltage domain to an I/O voltagedomain, e.g. 0 V to 1.98 V. The I/O voltage domain in this example ishigher than the core voltage domain, such that the first level shifter121 can shift the smoothed data signal from a low voltage state to ahigh voltage state. That is, the first level shifter 121 is a level upshifter in this example. The shifting operation at the first levelshifter 121 is controlled by a power-on-control (POC) signal POC1 123.The shifting operation will be performed upon a triggering of the POC1signal 123. That is, a state change (e.g. from a high voltage state to alow voltage state) of the POC1 signal 123 will enable the first levelshifter 121 to shift the smoothed data signal from the core voltagedomain to the I/O voltage domain, and to generate a level-up data signalI_up within the I/O voltage domain. The level-up data signal I_up is tobe sent or output to another chip via the I/O pad 190.

The I/O portion 120 in this example also includes a second level shifter122 that is coupled to the second buffer 112 and receives the smoothedOE signal from the second buffer 112. The second level shifter 122 canshift the smoothed OE signal from the core voltage domain to the I/Ovoltage domain, e.g. 0 V to 1.98 V. The I/O voltage domain in thisexample is higher than the core voltage domain, such that the secondlevel shifter 122 can shift the smoothed OE signal from a low voltagestate to a high voltage state. That is, the second level shifter 122 isa level up shifter in this example. The shifting operation at the secondlevel shifter 122 is controlled by a power-on-control (POC) signal POC2124. The shifting operation will be performed upon a triggering of thePOC2 signal 124. That is, a state change (e.g. from a high voltage stateto a low voltage state) of the POC2 signal 124 will enable the secondlevel shifter 122 to shift the smoothed OE signal from the core voltagedomain to the I/O voltage domain, and to generate a level-up OE signalOE_up within the I/O voltage domain. The level-up OE signal OE_up is tobe used to control the data output of the level-up data signal I_up toanother chip via the I/O pad 190.

The POC1 signal 123 and the POC2 signal 124 are two separate signalsthat can control the level shifting's of the first level shifter 121 andthe second level shifter 122, respectively. A gating circuit (not shownin FIG. 1), to be discussed in detail later, may be used to control asignal sequence of the POC1 signal 123 and the POC2 signal 124.Accordingly, the gating circuit can also control a signal sequence ofthe level-up data signal I_up and the level-up OE signal OE_up.Specifically, the gating circuit can control the level-up OE signalOE_up to be generated after the level-up data signal I_up is generatedand has reached to a stable logic state, to prevent glitch fromoccurring during a power ramp-up process of the circuit. During a powerramp-up process, the power of the I/O circuit and the power of the corecircuit are increased. This may happen when the integrated circuit onthe chip is turned on and starts working. After the power ramp-upprocess, the circuit begins normal operations. The POC1 signal 123 andthe POC2 signal 124 will not impact the normal operations of thecircuit, because they will be kept in a logic low state and enable thefirst level shifter 121 and the second level shifter 122 to worknormally.

The I/O portion 120 in this example also includes a control logiccircuit 126 that is coupled to the first level shifter 121 and thesecond level shifter 122 and is configured for driving the I/O pad 190to a voltage level based on the data signal I_up and the output enablesignal OE_up. That is, the control logic circuit 126 can output the datasignal I_up via the I/O pad 190 in response to the output enable signalOE_up. Specifically, when the output enable signal OE_up is notasserted, the control logic circuit 126 is in a tri-state mode and doesnot drive the I/O pad 190. When the output enable signal OE_up isasserted, the control logic circuit 126 drives the I/O pad 190 to avoltage level and/or a logical state corresponding to the data signalI_up.

Although not shown in FIG. 1, the circuit 100 may also include an inputbuffer that is in the I/O portion 120 to receive an input signal fromanother chip via the I/O pad 190 and drive the input signal into a leveldown shifter in the core portion 110. The level down shifter can shiftthe input signal from the I/O voltage domain to the core voltage domainfor the core circuit to receive the input signal with a proper voltagedomain.

During a power ramp-up process, two possible scenarios may happen in theI/O circuit. FIG. 2A illustrates an exemplary circuit behavior of theI/O circuit according to a first scenario during a power ramp-upprocess. As shown in FIG. 2A, the I/O power 202 of the I/O portion 120first increases to a high voltage state within the I/O voltage domainduring the power ramp-up process. The POC power 206 increases togetherwith the I/O power 202. Then, the core power 204 of the core portion 110increases to a high voltage state within the core voltage domain. Duringthe increasing process of the core power 204, the POC power 206 islowered to a low voltage state. In this example, one POC signal is usedto enable shifting operations at both the first level shifter 121 andthe second level shifter 122 upon a low voltage state of the POC power206. That is, in response to the low voltage state of the POC power 206,the first level shifter 121 and the second level shifter 122 start towork normally as level up shifters to shift up the voltage levels of theI_up signal 210 and the OE_up signal 220, respectively. Here, the samePOC signal serves as a gating signal of both the first level shifter 121and the second level shifter 122 before the powers are ready. There isno control of the time sequence of the POC signal entering the firstlevel shifter 121 and the second level shifter 122. Although generatedas one signal, the POC signal may enter the first level shifter 121 andthe second level shifter 122 at different timing points, such that theI_up signal 210 and the OE_up signal 220 may increase to a high voltagestate at different timing points.

For example, in the scenario shown in FIG. 2A, the I_up signal 210increases from the low voltage state L to the high voltage state H attime t1, while the OE_up signal 220 increases from the low voltage stateL to the high voltage state H at time t2 after time t1. In this case,there is no glitch occurring during the power ramp-up, as the pad power230 of the I/O pad 190 increases smoothly from a reference voltage stateZ to a high voltage state H together with the increase of the power ofthe OE_up signal 220. This is because the I_up signal 210 has reached astable high voltage state before the OE_up signal 220 increases to thehigh voltage state, such that when the OE_up signal 220 reaches the highvoltage state to assert, the pad power 230 of the I/O pad 190 will bedriven directly to a high voltage state according to the stable highvoltage state of the I_up signal 210.

FIG. 2B illustrates another exemplary circuit behavior of the I/Ocircuit according to a second scenario during a power ramp-up process.As shown in FIG. 2B, the I/O power 202 of the I/O portion 120 firstincreases to a high voltage state within the I/O voltage domain duringthe power ramp-up process. The POC power 206 increases together with theI/O power 202. Then, the core power 204 of the core portion 110increases to a high voltage state within the core voltage domain. Duringthe increasing process of the core power 204, the POC power 206 islowered to a low voltage state. Similar to the first scenario, in thisexample, one POC signal is used to enable shifting operations at boththe first level shifter 121 and the second level shifter 122 upon a lowvoltage state of the POC power 206. That is, in response to the lowvoltage state of the POC power 206, the first level shifter 121 and thesecond level shifter 122 start to work normally as level up shifters toshift up the voltage levels of the I_up signal 210 and the OE_up signal220, respectively. As discussed above, the same POC signal serves as agating signal of both the first level shifter 121 and the second levelshifter 122 before the powers are ready; and there is no control of thetime sequence of the POC signal entering the first level shifter 121 andthe second level shifter 122. Although generated as one signal, the POCsignal may enter the first level shifter 121 and the second levelshifter 122 at different timing points, such that the I_up signal 210and the OE_up signal 220 may increase to a high voltage state atdifferent timing points.

For example, in the scenario shown in FIG. 2B, the OE_up signal 220increases from the low voltage state L to the high voltage state H attime t1, while the I_up signal 210 increases from the low voltage stateL to the high voltage state H at time t2 after time t1. In this case,there is a glitch 250 occurring during the power ramp-up, as the padpower 230 of the I/O pad 190 glitches first and then increases from areference voltage state Z to a high voltage state H with the increase ofthe power of the I_up signal 210. This is because the I_up signal 210increases to the high voltage state after the OE_up signal 220 increasesto the high voltage state, such that when the OE_up signal 220 reachesthe high voltage state to assert, the pad power 230 of the I/O pad 190will not be driven directly to the high voltage state as the I_up signal210 still has a low voltage power. The OE signal in this case gates awrong I_up state into the I/O pad. Then when the I_up signal 210increases to the high voltage power at t2, the pad power 230 of the I/Opad 190 is driven to the high voltage state according to the highvoltage state of the I_up signal 210.

To avoid this glitch 250 as in FIG. 2B and ensure circuit behavior to bealways like the first scenario in FIG. 2A during a power ramp-upprocess, the present teaching discloses various embodiments of a gatingcircuit to generate two separate POC signals for controlling the firstlevel shifter 121 and the second level shifter 122 respectively and tocontrol the signal sequence of the two separate POC signals.

FIG. 3 illustrates an exemplary gating circuit 300 in an input/outputcircuit, e.g. the circuit 100 in FIG. 1, in accordance with someembodiments of the present disclosure. As shown in FIG. 3, the gatingcircuit 300 includes six transistors 341, 342, 343, 344, 345, 346, andtwo inverters 332, 334. The gating circuit 300 includes a firsttransistor 341 and a second transistor 342 (e.g., an n-type MOSFET and ap-type MOSFET, respectively) that are commonly gated by a logical invertof the first power-on-control signal POC1 310. In this example, as thelogical invert of the POC1 signal 310 is inverted again when enteringthe first transistor 341, the first transistor 341 is effectively gatedby the POC1 signal 310. The gating circuit 300 includes a thirdtransistor 343 that is coupled to the second transistor 342 and is gatedby a logical invert of the signal I 304. The gating circuit 300 includesa fourth transistor 344 (e.g., an n-type MOSFET) that is gated by thesignal I_up 306. The gating circuit 300 includes a fifth transistor 345(e.g., an n-type MOSFET) that is coupled to the fourth transistor 344and gated by the signal I 302.

The gating circuit 300 includes a first inverter 331 configured togenerate the second power-on-control signal POC2 320 based on outputs ofone or more of the six transistors. The gating circuit 300 also includesa second inverter 332 coupled between the first transistor 341 and thefirst inverter 331. The gating circuit 300 also includes a sixthtransistor 346 coupled to the fifth transistor 345 and gated by anoutput signal of the second inverter 332.

With this exemplary structure of the gating circuit 300, the secondpower-on-control signal POC2 320 is activated after the firstpower-on-control signal POC1 is activated. In one example, the POC1signal is first activated, i.e. changed from a high voltage state to alow voltage state. Accordingly, the logical invert of the firstpower-on-control signal POC1 310, as an input signal to the firsttransistor 341 and the second transistor 342, is changed from a lowvoltage state to a high voltage state. Then, the first transistor 341 isturned off and the second transistor 342 is turned on. As such, theinput of the second inverter 332 is changed from a high voltage state toa low voltage state. Then the input of the first inverter 331 is changedfrom a low voltage state to a high voltage state. Accordingly, theoutput of the first inverter 331 is changed from a high voltage state toa low voltage state, i.e. the second power-on-control signal POC2 320 isactivated. Therefore, the signal POC2 320 is activated after the signalPOC1 is activated. The third transistor 343, the fourth transistor 344,the fifth transistor 345, and the sixth transistor 346 can helpcontrolling the gating circuit 300. For example, by connecting thecontrol end of the sixth transistor 346 with the output of the secondinverter 332, the sixth transistor 346 enables the drain end of thefourth transistor 344 to be stabilized after the power ramp up process.

In the above example, once the signal POC1 is activated and changed to alow voltage state, the first level shifter 121 will start to worknormally as a level up shifter to shift up the voltage level of the I_upsignal 306. Similarly, once the signal POC2 is activated and changed toa low voltage state, the second level shifter 122 will start to worknormally as a level up shifter to shift up the voltage level of theOE_up signal. Because the structure of the gating circuit 300 ensuresthat the signal POC2 320 is activated after the signal POC1 isactivated, the OE_up signal is also ensured to be generated or shiftedup in response to the POC2 signal 320 after the data signal I_up isgenerated or shifted up in response to the POC1 signal, which preventsglitches from occurring during a power ramp-up process. According tovarious embodiments of the present teaching, one or more components(transistors or inverters) of the gating circuit 300 may be removed orreplaced without affecting the time dependence between the two POCsignals, such that the gating circuit 300 can still prevent glitchesfrom occurring during the power ramp-up process.

FIG. 4 illustrates an exemplary layout of a gating circuit, e.g. thegating circuit 300 in FIG. 3, in an input/output circuit, in accordancewith some embodiments of the present disclosure. As shown in FIG. 4, thelayout may include an N-type metal-oxide-semiconductor (NMOS) portion410 and a P-type metal-oxide-semiconductor (PMOS) portion 420, which isillustrated for example. The layout in FIG. 4 includes an oxidediffusion (OD) layer 430, a polysilicon (PO) layer 432 formed above theOD layer 430, and a metal over oxide (MD) layer 434 formed above the ODlayer 430. Different components of the gating circuit 300 in FIG. 3 aremarked at corresponding portions of the layout. For example, thetransistor 341 is implemented at the PMOS portion 420; while thetransistors 342, 343, 344, 345, 36 are implemented at the NMOS portion410. The first inverter 331 includes both an NMOS part 331.N implementedat the NMOS portion 410, and a PMOS part 331.P implemented at the PMOSportion 420. Similarly, the second inverter 332 includes both an NMOSpart 332.N implemented at the NMOS portion 410, and a PMOS part 332.Pimplemented at the PMOS portion 420. The layout further includes ametal-zero (M0) layer 436 formed on the PO layer 432 and the MD layer434; and includes a metal-one (M1) layer 438 formed on the M0 layer 436.Each of the M0 layer 436 and the M1 layer 438 includes metal linesconnecting the different components of the gating circuit. As shown inFIG. 4, an I/O power supply pin (VDDPST) is coupled to the M0 layer 436in the PMOS portion 420; while a ground reference pin (VSS) is coupledto the M1 layer 438 in the NMOS portion 410.

The logical complement of the POC1 signal 310 and the POC2 signal 320are controlled to have a specific time dependent relationship asdiscussed above, based on the exemplary layout shown in FIG. 4. In oneembodiment, the layout of the gating circuit merely occupies an areathat is less than a predetermined percentage (e.g. 0.5%, 1%, etc.) of atotal area of the I/O circuit. As such, the gating circuit has little orno impact on the implementation area of the I/O circuit. The layoutshown in FIG. 4 is just one example for implementing the gating circuit300 in FIG. 3, and other layouts may be used to implement the gatingcircuit 300 in FIG. 3 according to various embodiments of the presentteaching.

FIG. 5 illustrates another exemplary gating circuit 500 in aninput/output circuit, e.g. the circuit 100 in FIG. 1, in accordance withsome embodiments of the present disclosure. As shown in FIG. 5, thegating circuit 500 in this example includes a series circuit having aneven number of inverters 510, 520 connected in series. The seriescircuit receives the first power-on-control signal POC1 123 as an inputsignal and generates the second power-on-control signal POC2 124 as anoutput signal. The even number of inverters can provide a time delaybetween the POC1 signal 123 and the POC2 signal 124, and ensure aconsistent logic state between the POC1 signal 123 and the POC2 signal124. In one embodiment, the even number may be designed to be largeenough to ensure an enough time delay between the POC1 signal 123 andthe POC2 signal 124, such that the POC2 signal 124 is generated afterthe POC1 signal 123 has reached a stable logic state.

According to some embodiments of the present teaching, the I/O circuitincludes one or more additional gating circuits, each of which has astructure as that shown in FIG. 3 or FIG. 5. These gating circuits areconfigured to generate POC signals and control a signal sequence of allPOC signals based on a predetermined design, to ensure a desirabletiming sequence of signals in the I/O circuit. The desirable timingsequence may be for one or more signals other than the data signal andthe output enable signal.

FIG. 6 illustrates an exemplary circuit behavior during a power ramp-upof an input/output circuit, e.g. the circuit 100 in FIG. 1, inaccordance with some embodiments of the present disclosure. As shown inFIG. 6, the I/O power 602 of the I/O portion 120 first increases to ahigh voltage state within the I/O voltage domain during the powerramp-up process. The first POC power of POC1 606 and the second POCpower of POC2 608 increase together with the I/O power 602. In thisexample, two separate POC signals POC1 and POC2 are used to enableshifting operations at the first level shifter 121 and the second levelshifter 122 respectively, as shown in FIG. 1. That is, in response to alow voltage state of the POC1 signal 606, the first level shifter 121will start to work normally as a level up shifter to shift up thevoltage level of the I_up signal 610; and in response to a low voltagestate of the POC2 signal 608, the second level shifter 122 will start towork normally as a level up shifter to shift up the voltage level of theOE_up signal 620.

After the I/O power 602 increases to the high voltage state, the corepower 604 of the core portion 110 increases to a high voltage statewithin the core voltage domain. During the increasing process of thecore power 604, the POC power of POC1 606 is lowered to a low voltagestate, e.g. due to a control signal of the gating circuit. In responseto the low voltage state of the POC1 signal 606, the first level shifter121 starts to work normally as a level up shifter to shift up thevoltage level of the I_up signal 610. As shown in FIG. 6, after the POC1606 is lowered to the low voltage state, the I_up signal 610 increasesfrom the low voltage state L to the high voltage state H.

As discussed above, the gating circuit controls the time sequence forgenerating the two POC signals: the POC1 signal 606 and the POC2 signal608, such that the POC1 signal 606 is generated with a low voltage statebefore the POC2 signal 608 is generated with the low voltage state. Assuch, the POC1 signal enters the first level shifter 121 to enablegeneration of the I_up signal 610 before the POC2 signal enters thesecond level shifter 122 to enable generation of the OE_up signal 620.Here, the two POC signals serve as gating signals of the first levelshifter 121 and the second level shifter 122 respectively before thepowers are ready.

As shown in FIG. 6, after the I_up signal 610 reaches a stable highvoltage state H, the POC2 signal 608 is lowered to the low voltage stateto trigger the normal operation of the second level shifter 122. Then,the second level shifter 122 shifts up the voltage level of the OE_upsignal 620 from the low voltage state L to the high voltage state H. Asdiscussed above, this time dependent relationship between the POC1signal 606 and the POC2 signal 608 ensures that no glitch can occurduring the power ramp-up process of the I/O circuit. Specifically, inaccordance with this embodiment, the signal timing sequence of the I/Ocircuit includes the following in order: POC1 signal is activated bybeing decreased to a low voltage state, I_up data signal is activated bybeing increased to a high voltage state, POC2 signal is activated bybeing decreased to a low voltage state, and OE_up signal is activated bybeing increased to a high voltage state.

FIG. 7 illustrates an example of detailed circuit behaviors during powerramp-up of an input/output circuit, e.g. the circuit 100 in FIG. 1, inaccordance with some embodiments of the present disclosure. As shown inFIG. 7, the I/O power 702 first increases to a high voltage state withinthe I/O voltage domain during the power ramp-up process. The first POCpower of POC1 706 and the second POC power of POC2 708 follow the I/Opower 602 to increase to the high I/O voltage domain as well. In thisexample, two separate POC signals POC1 and POC2 are used to enableshifting operations at the first level shifter 121 and the second levelshifter 122 respectively, as shown in FIG. 1. That is, in response to alow voltage state of the POC1 signal 706, the first level shifter 121will start to work normally as a level up shifter to shift up thevoltage level of the I_up signal 710; and in response to a low voltagestate of the POC2 signal 708, the second level shifter 122 will start towork normally as a level up shifter to shift up the voltage level of theOE_up signal 720.

After the I/O power 702 increases to the high voltage state, the corepower 704 increases to a high voltage state within the core voltagedomain. During the increasing process of the core power 704, the POCpower of POC1 706 is lowered to a low voltage state, e.g. due to acontrol signal of the gating circuit. The circuit behaviors within theportion 790 can be seen in more detail in the corresponding enlargedview 792. As shown in the enlarged view 792, in response to the lowvoltage state of the POC1 signal 706, the voltage level of the I_upsignal 710 is increased up to the high voltage state. In this example,the first level shifter 121 shifts the data signal I 711 from a low corevoltage domain 0.75 V to a high I/O voltage domain 1.8 V to become thelevel-up data signal I_up 710.

As discussed above, the gating circuit controls the time sequence forgenerating the two POC signals: the POC1 signal 706 and the POC2 signal708, such that the POC1 signal 706 is generated with a low voltage statebefore the POC2 signal 708 is generated with the low voltage state. Assuch, the POC1 signal enters the first level shifter 121 to enablegeneration of the I_up signal 710 before the POC2 signal enters thesecond level shifter 122 to enable generation of the OE_up signal 720.Here, the two POC signals serve as gating signals of the first levelshifter 121 and the second level shifter 122 respectively before thepowers are ready.

As shown in the enlarged view 792, after the POC1 signal 706 is loweredto a low voltage state, the POC2 signal 708 is lowered to the lowvoltage state to trigger the normal operation of the second levelshifter 122. Then, in response to the low voltage state of the POC2signal 708, the voltage level of the OE_up signal 720 is increased up tothe high voltage state. In this example, the second level shifter 122shifts the OE signal 721 from a low core voltage domain 0.75 V to a highI/O voltage domain 1.8 V to become the level-up OE signal OE_up 720. Asdiscussed above, this time dependent relationship between the POC1signal 706 and the POC2 signal 708 ensures that no glitch can occur atthe pad voltage 730 during the power ramp-up process of the I/O circuit,because the level-up OE signal OE_up 720 is activated to reach the highvoltage state after the level-up data signal I_up 710 is activated toreach the high voltage state.

As shown in FIG. 7, the time duration from the voltage change of thePOC1 signal 706 to the voltage change of the POC2 signal 708 isapproximately 30 nanoseconds, which is much shorter than a typical timeduration of power ramp-up, e.g. 100 microseconds. As such, the disclosedtime sequence control the POC1 signal 706 and the POC2 signal 708 willnot impact the power ramp-up time of the I/O circuit.

FIG. 8 illustrates an exemplary block diagram of another circuit 800coupled to an input/output pad 890, in accordance with some embodimentsof the present disclosure. In accordance with one embodiment, thecircuit 800 may be part of a chip in an integrated circuit system. Thechip can communicate with other chips in the system via the I/O pad 890controlled by the I/O circuit 800. As shown in FIG. 8, the circuit 800includes two portions, a core portion 810 having a first voltage domainand an I/O portion 820 having a second voltage domain.

In this example, the core portion 810 includes a first buffer 811 thatreceives an input data signal I and generates a smoothed data signalwith a same voltage level as the input data signal I. The input datasignal I may be generated by a core circuit connected to the firstbuffer 811. The core circuit (not shown) performs a core function of thechip according to customer design. The core circuit generates the inputdata signal Ito output data to another chip via the I/O pad 890. Theinput data signal I generated by the core circuit has a voltage within acore domain, e.g. 0 V to 0.75 V.

The core portion 110 in this example also includes a second buffer 812that receives an output enable signal OE and generates a smoothed OEsignal with a same voltage level as the output enable signal OE. Theoutput enable signal OE is also generated by the core circuit connectedto the second buffer 812. The core circuit generates the output enablesignal OE to control the data output to another chip via the I/O pad890. The output enable signal OE generated by the core circuit has avoltage within the core domain, e.g. 0 V to 0.75 V.

The I/O portion 820 in this example includes a first level shifter 821that is coupled to the first buffer 811 and receives the smoothed datasignal from the first buffer 811. The first level shifter 821 can shiftthe smoothed data signal from the core voltage domain to an I/O voltagedomain, e.g. 0 V to 1.98 V. The I/O voltage domain in this example ishigher than the core voltage domain, such that the first level shifter821 can shift the smoothed data signal from a low voltage state to ahigh voltage state. That is, the first level shifter 821 is a level upshifter in this example. The shifting operation at the first levelshifter 821 is controlled by a POC signal 823. The shifting operationwill be performed upon a triggering of the POC signal 823. That is, astate change (e.g. from a high voltage state to a low voltage state) ofthe POC signal 823 will enable the first level shifter 821 to shift thesmoothed data signal from the core voltage domain to the I/O voltagedomain, and to generate a level-up data signal I_up within the I/Ovoltage domain. The level-up data signal I_up is to be sent or output toanother chip via the I/O pad 890.

The I/O portion 820 in this example also includes a second level shifter822 that is coupled to the second buffer 812 and receives the smoothedOE signal from the second buffer 812. The second level shifter 822 canshift the smoothed OE signal from the core voltage domain to the I/Ovoltage domain, e.g. 0 V to 1.98 V. The I/O voltage domain in thisexample is higher than the core voltage domain, such that the secondlevel shifter 822 can shift the smoothed OE signal from a low voltagestate to a high voltage state. That is, the second level shifter 822 isa level up shifter in this example. The shifting operation at the secondlevel shifter 822 is controlled by the POC signal 823 as well. Theshifting operation will be performed upon a triggering of the POC signal823. That is, a state change (e.g. from a high voltage state to a lowvoltage state) of the POC signal 823 will enable the second levelshifter 822 to shift the smoothed OE signal from the core voltage domainto the I/O voltage domain, and to generate a level-up OE signal OE_upwithin the I/O voltage domain. The level-up OE signal OE_up is to beused to control the data output of the level-up data signal I_up toanother chip via the I/O pad 890.

While the same POC signal serves as a gating signal of both the firstlevel shifter 821 and the second level shifter 822 before the powers areready, the I/O portion 820 also includes a delay circuit 825 that iscoupled to the second level shifter 822 and configured to generate adelayed OE_up signal based on the OE_up signal output by the secondlevel shifter 822. In this embodiment, the delay circuit 825 includes acapacitor C_OE coupled to an output of the second level shifter 822. Thecapacitor C_OE may be designed to have a capacitance that is largeenough to ensure that the delayed OE_up signal is generated after thedata signal I_up has reached a stable logic state. Accordingly, thedelay circuit 825 can control the level-up OE signal OE_up is delayed toenter the control logic circuit 826 with the high voltage state afterthe level-up data signal I_up enters the control logic circuit 126 witha stable high voltage logic state, to prevent glitch from occurringduring a power ramp-up process of the I/O circuit. In accordance withvarious embodiments, the delay circuit 825 may have a structuredifferent from that shown in FIG. 8 and still achieve a time delayeffect for the level-up OE signal OE_up. For example, the delay circuit825 can have a time delay effect, based on at least one of: a resistor,a capacitor, a transistor, a diode, and a timer.

The I/O portion 820 in this example also includes a control logiccircuit 826 that is coupled to the first level shifter 821 and thesecond level shifter 822 and is configured for driving the I/O pad 890to a voltage level based on the data signal I_up and the delayed OE_upsignal. That is, the control logic circuit 826 can output the datasignal I_up via the I/O pad 890 in response to the delayed OE_up signal.Specifically, when the delayed OE_up signal is not asserted, the controllogic circuit 826 is in a tri-state mode and does not drive the I/O pad890. When the delayed OE_up signal is asserted, the control logiccircuit 826 drives the I/O pad 890 to a voltage level and/or a logicalstate corresponding to the data signal I_up. Although not shown in FIG.8, the circuit 800 may also include an input buffer that is in the I/Oportion 820 to receive an input signal from another chip via the I/O pad890 and drive the input signal into a level down shifter in the coreportion 810. The level down shifter can shift the input signal from theI/O voltage domain to the core voltage domain for the core circuit toreceive the input signal with a proper voltage domain.

FIG. 9 is a flow chart illustrating an exemplary method 900 forpreventing glitch in a circuit coupled to an input/output pad, inaccordance with some embodiments of the present disclosure. At operation902, a first power-on-control (POC) signal is coupled to a first levelshifter in an input/output circuit. A first input signal is shifted atoperation 904 from a first voltage domain to a second voltage domain togenerate a data signal in response to the first POC signal. A second POCsignal is coupled at operation 906 to a second level shifter in theinput/output circuit. A second input signal is shifted at operation 908from the first voltage domain to the second voltage domain to generatean output enable signal in response to the second POC signal. Atoperation 910, an input/output pad is driven to a voltage level based onthe data signal and the output enable signal. The order of theoperations shown in FIG. 9 may be changed according to differentembodiments of the present disclosure.

In an embodiment, a circuit coupled to an input/output pad is disclosed.The circuit includes: a first level shifter, a second level shifter, anda control logic circuit. The first level shifter is configured forgenerating a data signal. The second level shifter is configured forgenerating an output enable signal. The first and second level shiftersare controlled by first and second power-on-control signals,respectively. The control logic circuit is coupled to the first levelshifter and the second level shifter, and configured for driving theinput/output pad to a voltage level based on the data signal and theoutput enable signal.

In another embodiment, a circuit coupled to an input/output pad isdisclosed. The circuit includes: a first level shifter, a second levelshifter, a delay circuit and a control logic circuit. The first levelshifter is configured for generating a data signal. The second levelshifter is configured for generating an output enable signal. The firstand second level shifters are controlled by a power-on-control signal.The delay circuit is coupled to the second level shifter and configuredto generate a delayed output enable signal based on the output enablesignal. The control logic circuit is coupled to the first level shifterand the second level shifter, and configured for driving theinput/output pad to a voltage level based on the data signal and thedelayed output enable signal.

In yet another embodiment, a method for preventing glitch in a circuitcoupled to an input/output pad is disclosed. The method includes:coupling a first power-on-control signal to a first level shifter in thecircuit; in response to the first power-on-control signal, shifting afirst input signal from a first voltage domain to a second voltagedomain to generate a data signal; coupling a second power-on-controlsignal to a second level shifter in the circuit; in response to thesecond power-on-control signal, shifting a second input signal from thefirst voltage domain to the second voltage domain to generate an outputenable signal; and driving the input/output pad to a voltage level basedon the data signal and the output enable signal.

The foregoing outlines features of several embodiments so that thoseordinary skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A circuit coupled to an input/output pad,comprising: a first level shifter configured for generating a datasignal, wherein the first level shifter is controlled by a firstpower-on-control signal; a second level shifter configured forgenerating an output enable signal, wherein the second level shifter iscontrolled by a second power-on-control signal; a gating circuitconfigured to generate the first and second power-on-control signals ina manner such that the first power-on-control signal enables the firstlevel shifter to generate the data signal before the secondpower-on-control signal enables the second level shifter to generate theoutput enable signal; and a control logic circuit configured to drivethe input/output pad to a voltage level based on the data signal and theoutput enable signal.
 2. The circuit of claim 1, wherein: the firstlevel shifter is configured to receive a first input signal at a firstvoltage domain and shift the first input signal to a second voltagedomain to generate the data signal in response to the firstpower-on-control signal; and the second level shifter is configured toreceive a second input signal at the first voltage domain and shift thesecond input signal to the second voltage domain to generate the outputenable signal in response to the second power-on-control signal.
 3. Thecircuit of claim 2, wherein the second voltage domain is higher than thefirst voltage domain.
 4. The circuit of claim 1, wherein the gatingcircuit is configured to control a signal sequence of the first andsecond power-on-control signals.
 5. The circuit of claim 1, wherein thegating circuit comprises: a first transistor gated by the data signal; asecond transistor gated by a signal that is a logical invert of thefirst power-on-control signal; and a first inverter configured togenerate the second power-on-control signal based on outputs of thefirst transistor and the second transistor, such that the secondpower-on-control signal is generated based on whether the data signalhas reached a stable logic state.
 6. The circuit of claim 5, wherein thegating circuit further comprises: a second inverter coupled between thesecond transistor and the first inverter; a third transistor coupled tothe first transistor and gated by an input signal of the first levelshifter; a fourth transistor coupled to the first transistor and gatedby a signal that is a logical invert of the input signal of the firstlevel shifter; a fifth transistor coupled to the second transistor andgated by the first power-on-control signal; and a sixth transistorcoupled to the first transistor and gated by an output signal of thesecond inverter.
 7. The circuit of claim 1, wherein: the gating circuitcomprises a series circuit having an even number of inverters connectedin series; the series circuit receives the first power-on-control signalas an input signal and generates the second power-on-control signal asan output signal; and the even number is large enough to ensure that thesecond power-on-control signal is generated after the firstpower-on-control signal enables the first level shifter to generate thedata signal.
 8. The circuit of claim 1, wherein the gating circuitoccupies an area that is less than a predetermined percentage of a totalarea of the circuit.
 9. The circuit of claim 1, further comprising anadditional gating circuit.
 10. The circuit of claim 9, wherein theadditional gating circuit is configured to generate a thirdpower-on-control signal and control a signal sequence of the first,second and third power-on-control signals based on a predetermineddesign.
 11. The circuit of claim 1, wherein the control logic circuit iscoupled to the first level shifter and the second level shifter.
 12. Acircuit, comprising: a first level shifter configured for generating adata signal; a second level shifter configured for generating an outputenable signal; a delay circuit coupled to the second level shifter andconfigured to generate a delayed output enable signal based on theoutput enable signal, wherein the delay circuit comprises a capacitorcoupled to an output of the second level shifter; and a control logiccircuit coupled to the first level shifter and the second level shifter.13. The circuit of claim 12, wherein: the first level shifter isconfigured to receive a first input signal at a first voltage domain andshift the first input signal to a second voltage domain to generate thedata signal in response to a first power-on-control signal; and thesecond level shifter is configured to receive a second input signal atthe first voltage domain and shift the second input signal to the secondvoltage domain to generate the output enable signal in response to asecond power-on-control signal.
 14. The circuit of claim 13, wherein thesecond voltage domain is higher than the first voltage domain.
 15. Thecircuit of claim 12, wherein the delayed output enable signal isgenerated after the data signal is generated.
 16. The circuit of claim12, wherein the capacitor has a capacitance that is large enough toensure that the delayed output enable signal is generated after the datasignal has reached a stable logic state.
 17. A method for preventingglitch in a circuit coupled to an input/output pad, comprising.: inresponse to a first power-on-control signal, shifting a first inputsignal from a first voltage domain to a second voltage domain togenerate a data signal; in response to a second power-on-control signal,shifting a second input signal from the first voltage domain to thesecond voltage domain to generate an output enable signal after the datasignal has reached a stable logic state; and driving the input/outputpad to a voltage level based on the data signal and the output enablesignal.
 18. The method of claim 17, wherein the second voltage domain ishigher than the first voltage domain.
 19. The method of claim 17,wherein: the first power-on-control signal is coupled to a first levelshifter in the circuit; the second power-on-control signal is coupled toa second level shifter in the circuit; the first power-on-control signalis not coupled to the second level shifter, and the secondpower-on-control signal is not coupled to the first level shifter. 20.The method of claim 17, further comprising: generating the firstpower-on-control signal; and generating the second power-on-controlsignal after a time duration starting when the first power-on-controlsignal is generated, wherein the time duration is within a power ramp-upprocess of the circuit.